The present invention relates generally to ophthalmic instruments, and more particularly to subjective ophthalmic refractors for evaluating refractive characteristics of a patient""s eye.
A subjective ophthalmic refractor typically comprises left-eye and right-eye batteries each having a defined viewing path along which an operator may selectively introduce combinations of testing lenses having known refractive properties. During examination, the patient is positioned in a darkened room with his or her eyes aligned to view a projected target chart along the viewing paths defined by the left-eye and right-eye batteries. The operator then performs well-known refracting procedures, including refraction using astigmatic charts and the Jackson cross-cylinder test. A goal of the examination procedure is to determine the sphere power, cylinder power, and cylinder axis of each eye so that a suitable pair of corrective lenses may be prescribed.
U.S. Pat. No. 2,968,213 describes an ophthalmic refractor of the prior art. FIG. 5 of the ""213 patent is an exploded view illustrating the internal components of a left eye lens battery, and serves to illustrate a longstanding arrangement for mechanically coupling a pair of rotatable cylinder lens carriers 8 and 9 and an associated ring-shaped cylinder power scale 50 of the battery to an adjustment knob 20 used by the operator to set a chosen cylinder power in the viewing path of the battery. As can be seen at FIG. 1 of the ""213 patent, the indicia on scale 50 can be viewed by the operator through an display window or opening 52 in the battery housing. Typically, the indicia are numerical cylinder power values from 0 to 6.00 diopters in quarter-diopter increments, and are angularly spaced at regular angular increments about a central axis of the ring-shaped scale. The mechanical interconnections from knob 20, through cylinder power scale 50, to the lens carriers 8 and 9 are designed such that rotation of knob 20 positions two lenses (or a lens and an empty lens cell), one from lens carrier 8 and one from lens carrier 9, in series in the viewing path to produce a resultant cylinder power. Lens carrier 8 is a xe2x80x9cweakxe2x80x9d cylinder lens carrier having, for example, a blank lens cell (zero power)and four cylinder lenses ranging in power from 0.25 diopters to 1.00 diopters at quarter diopter increments. Meanwhile, lens carrier 9 is a xe2x80x9cstrongxe2x80x9d cylinder lens carrier having, for example, a blank lens cell (zero power) and four cylinder lenses ranging in power from 1.25 diopters to 5.00 diopters at 1.25-diopter increments. Consequently, by indexing the weak cylinder lens carrier 8 five times for every one index movement of strong cylinder lens carrier 9, a cylinder power range of 0.00 diopters to 6.00 diopters at quarter-diopter increments is possible in agreement with the indicia on scale 50.
With continued reference to U.S. Pat. No. 2,968,213, it will be seen that adjustment knob 20 drives a shaft 21 having at its opposite end a dual gear comprising a small front gear 23 and a larger rear gear 25. Larger gear 25 meshes with a gear 31 which is fixedly and permanently attached to driver plate 27 in coaxial arrangement therewith, whereby rotation of knob 20 and larger gear 25 produces counter-rotation of driver plate 27. As driver plate 27 rotates about its axis, four short pegs. 39a-39d on the driver plate successively engage weak cylinder lens carrier 8 to index only the weak lens carrier, and a fifth longer peg 40 engages both the weak and strong lens carriers 8 and 9 to index both carriers, in the manner of a Geneva mechanism. A spring-biased roller 48xe2x80x2 cooperates with five circumferential detents 53xe2x80x2 in driver plate 27 to allow the operator to feel each index position at adjustment knob 20. A locking plate 37 having a recess 49 is fixedly mounted for rotation with driver plate 27 by a hub 33 and cooperates with a star wheel 47 on strong cylinder lens carrier 9 to prevent rotation of the strong cylinder lens carrier except when it is time for the strong cylinder lens carrier to index.
Meanwhile, small front gear 23 meshes with internal gear teeth on ring-shaped scale 50 to rotate the scale in coordination with the indexing of cylinder lens carriers 8 and 9, whereby an appropriate cylinder power value marked on the scale appears through display window 52. Scale 50 is constrained both radially and axially by three bearings 56 located at respective positions about the circumference of scale 50.
During manufacturing assembly of an ophthalmic refractor formed in accordance with the ""213 patent, it is necessary to carefully align cylinder power scale 50 such that a correct cylinder power value is precisely centered in display window 52 while dependent rotational positions of driver plate 27 and locking plate 37 are also in proper alignment for functioning of the Geneva mechanism. However, because an exact locational relationship between the internal gear teeth and the markings on scale 50 is not specified and one scale differs slightly from another in this regard, the task of centering the scale markings in the display window and achieving proper rotational alignment among the interconnected parts has involved a process of trial and error. For example, different scales 50 and other component parts in the mechanical interconnection are tried from production batches of these parts until a suitable combination of parts produces acceptable rotational alignment and centering of the scale markings.
FIGS. 2 and 3 herein help illustrate assembly of an ophthalmic refractor eye battery 111A according to the prior art. A cylinder power scale 118 is mounted in eye battery housing 112A, and adjustment knob shaft 124 is inserted through a provided bearing opening in the housing such that scale drive gear 126 (see small front gear 23 of the ""213 patent) fixed to the shaft meshes with internal gear 130 of cylinder power scale 118. A driver plate 127 having a gear 131 permanently fixed thereto and a locking plate 133 having a hub 135 permanently fixed thereto are fastened together in a predefined angular relationship such that an arcuate recess 159 of the locking plate is centered with respect to the longer peg 160 of the driver plate. More specifically, three angularly spaced fastener holes 145 are provided through hub 135 in set relation to recess 159 and three corresponding threaded holes 147 are provided through gear 131 in set relation to longer peg 160, and the hub 135 is fastened to gear 131 by threaded fasteners 149 operable through access openings 151 to form a subassembly 153 comprising locking plate 133, hub 135, driver plate 127, and gear 131. This subassembly 153 is then mounted on hub stem 137 such that gear 131 meshes with a larger gear 128 (see larger rear gear 25 of the ""213 patent) fixed to adjustment knob shaft 124 adjacent scale drive gear 126. A spring-biased roller (not shown in FIGS. 2 and 3 but equivalent to spring-biased roller 48xe2x80x2 shown in the ""213 patent) engages an appropriate detent 155 in driver plate 127 to set the relationship of subassembly 153 to a star wheel and lens carriers (not shown in FIGS. 2 and 3 but equivalent to star wheel 47 and lens carriers 8 and 9 of the ""213 patent). Consequently, in order for gear 131 of subassembly 153 to mesh with larger gear 128, the adjustment knob shaft may have to rotate slightly, thereby rotating the scale drive gear 126 and scale 118 and producing an xe2x80x9cuncenteredxe2x80x9d appearance of the corresponding scale marking in display window 122. To correct this, different scales 118, driver plates 127 with attached gear 131, and locking plates 133 with attached hub 135 must be tried until a satisfactory centering is achieved.
Understandably, this assembly process is time consuming and requires skill and patience on the part of the assembly technician. The introduction of tighter tolerances for the component parts can alleviate the problem to some extent, however this introduces increased costs associated with manufacturing the component parts.
Therefore, it is an object of the present invention to increase efficiency in the assembly process for manufacturing subjective ophthalmic refractors.
It is another object of the present invention to improve the precision with which cylinder power scale markings are centered in a display window of a subjective ophthalmic refractor for higher quality from one refractor to the next.
It is another object of the present invention to diminish the level of skill and patience required of an assembly technician in assembling a subjective ophthalmic refractor.
It is a further object of the present invention to achieve the objects stated above without introducing more exacting tolerances with respect to component parts of the ophthalmic refractor.
In furtherance of these and other objects, an ophthalmic refractor of a type comprising a cylinder power adjustment knob operatively connected to a drive assembly for indexing at least one rotatable lens carrier and to a corresponding rotatable cylinder power scale is made easier to assemble at a higher level of quality with respect to centering of scale markings in a display window. More specifically, the drive assembly can be set in a required rotational orientation separately from and after the centering of an appropriate scale marking in a display widow of the eye battery housing without affecting the centered position of the scale marking. In accordance with a preferred embodiment, the cylinder power adjustment knob is coupled to a shaft that carries a pair of gears at its distal end for rotation with the adjustment knob, a first gear for transmitting torque to the cylinder power scale and a second gear for transferring torque to a Geneva mechanism drive assembly and the other gear. The second gear meshes with a third gear that is coaxial with a driver plate of the Geneva mechanism. A gear phasing hub is adjustable axially relative to the third gear to selectively clamp the driver plate to the third gear at any necessary rotational orientation. An end cap threadably received by the gear phasing hub secures a locking plate of the Geneva mechanism to the, gear phasing hub in a proper rotational orientation relative to the driver plate. Thus, both the driver plate and locking plate can be set without disturbing the centered condition of the cylinder power scale.